Professor J. Klein

Research

Our group studies the physics and physical chemistry of soft matter, including simple liquids, polymers and bio-macromolecules. In particular, we study their behaviour and interactions at surfaces, interfaces and in confined geometries. We employ a variety of experimental techniques, with emphasis on direct ångstrom level measurements of forces between atomically-smooth surfaces, and between adsorbed or confined molecules. We have developed force-measuring methods with resolution and sensitivity some 3 - 4 orders of magnitude better (per molecule) than scanning probe techniques (e.g. AFM), and use these to probe both dynamics and equilibrium properties. Some recent projects are outlined below.

1. Liquids confined to nanometre-sized pores

The properties of liquids confined to gaps or pores of molecular dimensions are very different to those in bulk, due to the different balance between entropy and enthalpy in ultra-small volumes, and to the effects of surface fields. This is especially important for understanding lubrication phenomena and properties of thin films or membranes. Using a surface-force balance (SFB) we probe directly the liquid-to-solid transitions induced by increasing confinement, and the solid-like properties of such constrained 'liquids'.

2. Wetting and dewetting

Liquid films on non-wetting substrates are common in areas from detergency to microlithography and in devices where optically- or electronically-active organic films are used. Such films may spontaneously rupture and de-wet as a results of long-ranged van der Waals fields. In the case of liquid mixtures phase separation may additionally occur, modifying the film breakup and its final structure. Using ellipsometry, computerised video-microscopy and nuclear-reaction analysis we investigate the behaviour of model polymeric liquids - where molecular time and spatial scales are conveniently enhanced - and relate them to microscopic models.

3. Nanotribology

The study of friction at the nanometre scale (nanotribology) forms an important part of our research. Using SFBs we investigate the way in which polymer molecules can modify friction between surfaces to which they are attached, of direct relevance to biolubrication where (polymeric) articular cartilage surfaces rub past each other. We particularly examine at the molecular level the role of configurational entropy (and the resulting steric interactions) in reducing friction between polymer layers, and the interactions between charged molecules and of counterions which are ubiquitous in biological systems.

4. Surfactants and boundary lubrication

Surfactants are ubiquitous in both technological applications and in living systems, where they act to compatibilise surfaces and also as boundary lubricants. We use custom-synthesized surfactants to examine, with unique sensitivity, the interactions and particularly the mechanism of boundary lubrication in aqueous systems, using at the same time neutron reflectometry (with R.K. Thomas of the department) to determine their molecular-level structure on the surfaces.

5. Chiral recognition in biomaterials

Recent evidence suggests that cell-surface interactions may be controlled at the level of chiral recognition a cell may adhere to a given surface but not to its enantiomorphic counterpart. To get better insight into this we are studying, using ultra-sensitive SFBs, the interactions of cell-coating biopolymers with different chiral groups.